When correctly applied, joint efficiency determines the required thickness of a vessel component. This article concerns the principles applied by the ASME Section VIII, Division 1 pressure vessel code, because it is the most widely used code worldwide.

The application of joint efficiency is often misunderstood, and results in violations of the code, and even costly rework. This article attempts to clear away some of the misconceptions. We start off by defining the meaning of joint efficiency, denoted by the letter ‘E.’

That definition cannot be found anywhere in the code, but if you think about it, that must be the definition. We know the specified strength of the parent plate, because we can get it directly from the code (ASME Section II-Part D). But, how can we assess the strength of the weld metal? Actually, the code uses another method, but first we need to know a little more before we can apply a joint efficiency.